Photosensitive urethane liquid resin (brand name "APR," product of Asahi Chemical Industry Co., Ltd.) which is preservable in a liquid state but is curable when subjected to an external stimulus such as irradiation with ultraviolet rays has been recently used for preparing printing plates for printing newspapers and producing prints on corrugated boards.
Such printing plates are prepared by the steps shown in FIGS. 8 to 11.
The conventional process will be described for preparing a printing plate 1 for printing on a corrugated board, for example, the mark "AB" as shown in FIG. 6. (The mark appears as a reversed mark on the surface of the printing plate.)
FIG. 7 shows a negative film 3 and a masking sheet 4 for use in preparing the printing plate 1. A light transmitting pattern 31 corresponding to the mark is formed on the negative film 3. A surface for completely blocking light is formed over the hatched portion of the negative film 3 except the pattern 31.
The masking sheet 4 comprises a transparent film and a light blocking film removably affixed to the film. The light blocking film is cut out to form a light transmitting pattern 41 corresponding to and slightly larger than the light transmitting pattern 31 on the negative film 3. The other portion of the light blocking film indicated by hatching has a surface for completely blocking light.
Step a: The negative film 3 is placed on a transparent base plate 5, and a protecting transparent film 6 is placed over the negative film 3 as shown in FIG. 8.
Step b: A frame 7 serving as a barrier for a photosensitive liquid resin is placed along the outer periphery of the transparent film 6 to surround the entire negative film 3. The liquid resin is poured onto the transparent film 6 inside the frame 7 to form a photosensitive liquid resin layer 2 (FIG. 8).
Step c: A transparent base film 11 is placed over the surface of the liquid resin layer 2, and the masking sheet 4 is placed over the base film 11 (FIG. 8).
Step d: The resulting assembly is irradiated by ultraviolet lamps 51, 51 from outside the masking sheet 4 and the transparent base plate 5 to expose the liquid resin layer 2 to the light as shown in FIG. 9. In corresponding relation to the patterns 41, 31 of the masking sheet 4 and the negative film 3, the exposed portions of the resin layer start curing, while the unexposed portions remain liquid. The cured resin portion toward the negative film 3 becomes a relief portion 21 conforming to the mark. The cured resin portion toward the masking sheet 4 makes a mount portion 22 continuous with the relief portion 21 and having an increased width to reinforce the portion 21.
Step e: The masking sheet 4 is removed, and the resin layer is exposed to the light again for a short period of time from outside the base film 11 as seen in FIG. 10 to cure the entire upper surface of the resin layer to a small thickness and form a base portion 23 which is joined with the base film 11.
Step f: The frame 7 and the transparent film 6 are removed, and the uncured liquid resin is collected.
As shown in FIG. 11, each relief portion 21 is reinforced by the mount portion 22, which is continuous with the base portion 23 covering approximately over the entire surface of the base film 11. Thus, the printing plate 1 is formed.
The printing plate 1 has a double-layer structure in which the resin base portion 23 uniformly covers the base film 11. The printing plate 1 is cut to a suitable size including the relief portions 21, the mount portions 22 and matching marks 12, fixed in position to a transparent mount film 8 for a printing machine and then removably attached to the rotary drum of the printing machine.